Just FYI:
- Not setting TearFree does not make a difference, it still hits the same
failed assert.
- Starting kwin_x11 and plasmashell from a plain x shell does not trigger the
crash. startkde instead triggers the crash

Created attachment 133705[details]
another log with failed assert
Attaching Xorg.0.log, xorg-server compiled with --enable-debug=full. This looks similar to what's happening in this bug, except that assert does not fail on Plasma load, but rather non-deterministically some time later.